A points system used on the Xbox 360. Often abbreviated as just 'G'. Every Xbox 360 game has "achievements" and when you unlock these achievements you will get a certain amount of gamerscore.
For example, one achievement on Gears of War is called "Commando" and you unlock this when you complete the game on the hardest difficulty. It is worth 30G.
Your Gamerscore is shown on your Gamercard.

Gamerscore is a way of measuring a gamer’s progress of games on the Xbox 360. Games award you with various achievements for completing certain tasks (i.e. kill 10 enemy’s in 10 seconds gives you 50 gamerscore).

You will always be made fun of for your gamerscore, either for being real low or real high. This is how many people see a person’s gamerscore:

0 – 1000: Really low, why do you even own an Xbox? Your a noob.
1000 – 2500: Low, there is more to play than Halo and GTA dude. Still a noob.
2500 – 5000: Below Average, Red Ring of Death strikes here. Emerging from noobness.
5000 – 7500: Average, you’re a gamer but you don’t run it completely into the ground. Your no longer a noob.
7500 – 10000: High, and Impressive.
10000 – 15000: Quite High, at this point you may start being called a ‘nerd’.
15000 – 20000: Damn Son, at this point you are a nerd.
20000 – 30000: Holy Fuck, its called outdoors, you know with trees and animals…
30000 – 50000: No Life, you live in your parents basement, no job, no girlfriend, ect.
50K and Up: Virgin, you have never seen a pussy, and at this rate you never will.

I saw someone on Xbox Live who had a gamerscore of 110,000, and couldn't believe that someone could possibly have that much spare time.

The sum of achievements you've earned on your Xbox 360 account. The way most people feel about gamerscore is if someone has a lower gamerscore than you, then they are a noob, and if someone has a higher gamerscore than you, then they don't have a life.

People with low gamerscores typically don't care about their gamerscore, or they only play Call of Duty. Likewise, people with high gamerscores typically care about their gamerscore and play a ton of games just to raise it.

A feature added into games upon the release of the Xbox 360. It was intended to be a bit of fun, but because Xbox Live is just the huge cuntfest that it is, it is now a number used to criticise other people.

If for example, someone has a low gamerscore they suck and should immediately give up on living. However, if someone has a high gamerscore then they have no life. These are not the only two parts of the spectrum though, because your gamerscore is the perfect balance of skill while still having a life.

I have a gamerscore of 77,770. Therefore everyone with 77,769 gamerscore or less sucks at playing video games. Anyone,with 77,771 gamerscore or more has no life.